<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="js/jquery-3.5.1.js"></script>
</head>
<body>
<input type="text" id="username" />
<script type="text/javascript">
    $(function (){
        let input = document.querySelector("#username") ;
        input.value = "执行方法" ;
    })  //页面加载完成之后,实现方法
</script>
</body>
</html>